1. Field of the Invention
This invention relates to an audio motion monitor, and particularly a monitor for sensing the motion of a person, such as an infant or adult in a bed or crib, or a child wearing a toy costume, or an object, such as a toy, using the modal noise produced by transmitting a coherent light signal through an optical fiber.
2. Discussion of Background
Several companies make and sell audio monitors for use with infants and children. These monitors operate by means of a one-way transmission of sound using a microphone/transmitter, and typically include a base unit located close to the child to detect sounds in the area and transmit the detected sounds via a radio to a remote monitor. Typically, the remote monitor is a portable battery operated unit which can be worn on the belt of a care giver or otherwise conveniently kept near to the care giver. The conventional infant monitors are listening devices by which sounds of crying or other activity by the child can be routinely monitored remotely by the care giver.
The above-identified U.S. patent application Ser. Nos. 07/802,868 and 07/975,853, each use the modal noise induced in coherent light propagating in a multi-mode optical fiber in the presence of mechanical motions of a human subject to detect the subject's breathing and heartbeat. These two applications teach injecting a coherent light beam into one end of a multi-mode optical fiber and illuminating a photodetector, optimally a two-segment "split" detector, with the light emerging from the other end of the optical fiber, and processing the signals derived from the modal noise existing at the output end of the fiber to detect and quantify the subject's breathing and heartbeat. However, under certain circumstances, it may not be necessary to employ the processing necessary to detect and quantify the subject's heartbeat and breathing, and for such circumstances a simpler, lower cost approach is desirable.